The simplest solar cells have contacts on the front and rear surfaces to collect the negative and positive charge carriers. However the screen-printed metal comprising the front-side contacts blocks a significant area from receiving sunlight, often referred to as ‘shadowing’. Some newer architectures have been proposed to address this. One such example of such devices are metal wrap through (MWT) devices, where the thin metal ‘fingers’ are moved to the rear surface as well, leaving the front with much less metal. This is made possible by drilling tiny vias to connect the front surface with rear-surface contacts. With MWT, this requires about 8-200 holes per wafer.
A schematic representation of part of a prior art conventional bulk crystalline silicon solar cell is depicted in FIG. 1. See Kerschaver et al. “Back-contact Solar Cells: A Review” Prog. Photovolt: Res. Appl. 2006; 14: 107-123. The silicon base 101 is the main part of the mechanical structure. The emitter 102 is located near the top or front surface. A metal grid 103, 104 to extract the carriers from the device contacts each of these silicon regions. Whereas the rear surface is often fully covered by a base contact 105 (as in the drawing), on the front surface the metal grid is the result of a trade-off between having low coverage to limit optical losses and high coverage to limit resistive losses. Most manufacturers apply a front grid consisting of thin parallel lines 103 (fingers) that transport the current to centrally located busbars 104. The busbars are relatively wide and can be used as solder pads for connecting to external leads.
The contact wrap-through or metallization wrap-through (MWT) back-contact cell is the concept that is most closely linked to the conventional cell structure. In these cells, the emitter is located near the front surface, but part of the front metallization grid is moved from the front to the rear surface. In the schematic representation of the prior art cell in FIG. 2, this is depicted as the busbar 104 moving from one surface to the other. The remaining front surface grid 103 is connected to the interconnection pads 107 on the rear surface by extending it through a number of openings 106 in the wafer. The base contact 105 is electrically isolated from the interconnection pads 107 as shown in FIG. 2. See Kerschaver et al. “Back-contact Solar Cells: A Review” Prog. Photovolt: Res. Appl. 2006; 14: 107-123.
The MWT cells provide advantages by moving the front bus bar to the back—the shading losses are minimized, with a resulting increase in cell efficiency. However, in addition to performance optimization, there is a need for new back-contact cell/module designs that can make use of new assembly technologies that are inherently more scaleable (i.e., larger and/or thinner cells) with improved cost/throughput compared to current assembly processes using conventional cells. In particular, there is a need for new MWT cell designs and fabrication methods that are compatible with thin epitaxial silicon solar cells.